Field of the Invention
The invention relates to a method for operating a wind turbine that supplies power. The wind turbine is operated with variable rotational speed between a predeterminable minimum rotational speed and a predeterminable maximum rotational speed, wherein a characteristic variable of an oscillation of the wind turbine or of a part of the wind turbine is detected. The invention further relates to a wind turbine having a tower and a rotor. Further, a generator with variable rotational speed is provided and an open-loop or closed-loop control device provides open-loop control or closed-loop control of the rotational speed of the rotor between a minimum rotational speed and a maximum rotational speed during a power-supplying operation of the wind turbine. A sensor for detecting a characteristic variable of an oscillation of the wind turbine or of a part of the wind turbine is also provided.
In the case of wind turbines operating with variable rotational speed, a rotor excitation end very often lies close to a tower natural frequency, which can lead to resonance oscillations under unfavourable boundary conditions, in the case of the lower operating rotational speed, which can also be called the minimum rotational speed. Normally, in order to avoid these resonance oscillations, the tower natural frequency is determined by measurement and the rotational speed range of the wind turbine is set such that operation is guaranteed with a sufficiently low oscillation level under all conditions. For this, the permissible minimum rotational speed is frequently determined such that the excitation end is 5% to 10% above the tower frequency. But this leads to profit cuts because the rotational speed range is restricted and the rotor can thus only be operated in a restricted wind speed range with optimal rotational speed and thus optimal degree of efficiency.
Description of Related Art
WO 01/77524 A1 discloses a tower-natural-frequency-dependent operating control of offshore wind turbines, in which the respective critical natural frequencies of the system and/or of system parts are determined. The rotational speed range of the rotor, in which an excitation of the overall system and/or individual system parts takes place in the area of the critical natural frequencies, is determined. The wind turbine is only operated below and above the critical rotational speed range with quick passage through the critical rotational speed range.
EP 1 816 347 A1 discloses a wind turbine, which is controlled such that when an oscillation occurs, components of the wind turbine are activated such that they can act against the oscillation excitation.